The campaign has had a great influence on young people. A lot of them have stopped smoking.

So influential is the campaign that a lot of young people have stopped smoking.

Butfor thegreatinfluenceofthecampaign, alotofyoungpeople wouldhavestoppedsmoking.

Notuntilalotof youngpeoplehadstoppedsmokingdidthecampaigngreatlyinfluencethem.

Only if the campaign greatly influenced young people would a lot of them stop smoking.